Laser apparatuses supply appropriate energy to a material such that electrons can release photons and generate laser light by using mirrors such that the photons pass through the material repeatedly. Such laser apparatus has continued to grow up according to background of the remarkable advance in the field of optical communication, increase of demand for industrial laser, growth of medical applications, etc. In recent years, optical fiber laser has been developed and utilized in various fields. The optical fiber laser is continuously expanding its application fields by virtue of its advantages such as excellent characteristics of laser beam, superior reliability and ease of maintenance.
On the other hand, it is demanded for pulsed laser having narrow line width, in order for light input to the laser apparatus to be output as light of a different wavelength through wavelength conversion. For example, in order to utilize the optical fiber laser as high quality laser within the ultraviolet region, which can be utilized as a light source for an ultra-fine laser process system, it is demanded for the pulsed fiber laser having narrow line width, which is efficient in wavelength conversion.
Photonic crystal fiber of which core size is several tens of μm or Large-Mode-Area Double Clad Fiber (hereinafter, it is referred to as “LMA DCF,” or “double-clad optical fiber”) is used as the optical fiber. The Photonic crystal fiber has an advantage of having excellent beam quality. However, it is not possible to configure high output all-fiber laser because connection between the optical fibers is difficult. On the contrary, since the LMA DCF allows connection between the optical fibers to be made easily by means of a large-diameter optical fiber connector, it is easy to make optical fiber laser in a form of all-fiber optical fiber. As for the structure of the LMA DCF, it is, in general, composed of a core of several tens of μm, an inner clad of several hundreds of μm, and an outer clad surrounding those components.
Optical fiber structure of the LMA DCF is now described. First, the core is doped with a rare earth metal and its numeric aperture (NA) is approximately 0.04 to 0.08. The inner clad serves to propagate excitation light and its NA is approximately 0.4 to 0.5. The outer clad may be made from acrylic material having a lower refractive index than the inner clad. By virtue of such structure, excitation light does not exit to the outside and propagates along the optical fiber while it is guided by the inner clad. At this time, excitation light propagating along the optical fiber is absorbed by the rare earth ion doped to the core and energy absorbed thereby can amplify the seed light.
As for the LMA DCF optical fiber, diameter of the inner clad corresponds to approximately 5 to 20 times that of the core. The excitation light being propagated through the inner clad is absorbed in the region where the inner clad is overlapped with the core over, i.e., the excitation light passes through the core. However, since the outside of the core is not doped, light absorption does not occur in the outside of the core. Therefore, when the excitation light is distributed in the inside of the inner clad, light absorbance of the excitation light per unit length is lowered compared to a case where the excitation light is forced to proceed into the core and thereby distributed only in the core. In this case, absorbance of the excitation light when the excitation light propagates through the core of the optical fiber is represented as “core absorbance,” while absorbance of the excitation light when the excitation light propagates through the inner clad of the optical fiber is represented as “clad absorbance.” The clad absorbance per unit length of the optical fiber may vary depending on the structure of individual optical fiber, but it is lowered substantially by a reciprocal number of area ratio compared to the core absorbance. For example, if the diameter of the inner clad is 5 to 20 times larger than the diameter of the core, the clad absorbance is lowered by approximately 25 to 400 times the area ratio.
In the LMA DCF optical fiber, there is a problem that if the length of laser medium is short, efficiency is lowered because the excitation light cannot be absorbed sufficiently, whereas if the length of laser medium is long, efficiency is lowered because signal light is rather absorbed. Taking into consideration of such problem, a typical optical fiber laser is generally designed such that 90% of the excitation light is absorbed.
In the optical fiber laser as discussed above, a method of amplifying low power seed beam to high output through an amplifier is generally used in order to obtain high power laser with high peak power.
In order to generate high power laser beam in the optical fiber laser, it is required to enlarge diameter of the optical fiber. However, if the diameter of the optical fiber is enlarged, there is a disadvantage that spatial mode of the laser beam is generated in a multimode, not in a single mode. Therefore, the size of the core of the optical fiber cannot be increased indefinitely in order to maintain quality of the output beam in a single mode, whereby the maximum output of the optical fiber laser is limited.
In order to generate effective laser beam in the optical fiber laser, it is required to adapt the length of the optical fiber so as to absorb the excitation light sufficiently. However, if the length of the optical fiber is lengthened in the high power optical fiber laser, output is restricted due to nonlinear phenomenon such as stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS). In general, threshold value of light output of the SBS is given in the form as follows:
      P    th    SBS    -            21      ⁢                          ⁢              A        SBS                            g        eff            ⁢              L        eff            
Where, ASBS represents an effective cross-section area that optical fiber reacts with fiber laser beam, geff represents an effective gain coefficient of the SBS, and Leff represents an effective length of optical fiber. Furthermore, threshold value of the SRS is given in the form as follows:
      P    th    SRS    -            16      ⁢                          ⁢              A        R                            g        R            ⁢      L      
Where, AR represents an effective cross-section area, gR represents Raman gain coefficient, L represents length of optical fiber. Accordingly, in order to suppress generation of the SBS and SRS in the high power optical fiber laser, it is required to enlarge the cross-section area of the optical fiber as large as possible and reduce the length of the optical fiber. However, as already described, if the diameter of the core of the optical fiber is enlarged to 100 μm or more, there is a problem that a single mode cannot be generated, whereby quality of the laser is deteriorated.
As a way to obtain high power fiber laser while, if circumstances allow, reducing the length of the optical fiber and limiting the diameter of the optical fiber below a certain level, it is possible to contemplate a way to increase absorbance of the excitation light per unit light. In other words, it is possible to increase doping concentration of dopant ions in the optical fiber. However, since if the doping concentration becomes high, photodarkening effect or the like becomes strong, there is occurred a problem that using such optical fiber as laser gain medium is inappropriate.
In the LMA DCF optical fiber, as a way to obtain high output optical fiber laser under the condition that length and diameter of the optical fiber and concentration of dopant ions in the optical fiber are restricted, it is possible to contemplate a way to allow the excitation light to propagate to the core so as to enhance light absorbance in the optical fiber. This case requires a small light source allowing NA of the excitation light to be smaller than NA of the core of the optical fiber and an optical coupler for coupling excitation light and signal light together. However, since the NA of the output beam is in the range between 0.16 and 0.4 in an optical fiber-coupled type of a high output laser diode used as an excitation light source, of which output is 10 watts or more, it is not possible to couple the excitation light to the core so as to be guided in the core. Furthermore, in the LMA DCF, there is also a problem that it is not possible to use an optical coupler that can couple high power excitation light to signal light.